1. Field of the Invention
This invention relates to the recovery of hydrocarbons from a subterranean formation containing oil-bearing and water-bearing zones which have been treated with crosslinkable polymer to remove the gel from the subterranean formation. Permeability may be selectively restored in the oil-bearing zones in the formation.
2. Brief Description of the Prior Art
During the recovery of hydrocarbons from subterranean formations containing oil-bearing and water-bearing zones, a number of problems are encountered. For instance, during drilling operations zones may be encountered which require the use of a crosslinked polymer to restrict the flow of fluids from the zones. Such instances can occur during drilling when high-fluid-loss zones are encountered. In such instances, a general treatment of the entire formation with a crosslinkable polymer to form a gel to shut off the flow of fluids is desirable. Such problems can be encountered in water injection wells where naturally-occurring or created fractures exist in the formation and the like. Generally speaking the use of the crosslinkable polymers is effective to plug the entire formation area. In some instances, this is desirable. In other instances it is desirable to be able to produce hydrocarbons from the oil-bearing zones of the subterranean formation when the well is completed. In such instances it is necessary to remove the plugging polymer gels from the subterranean formation.
Suitable polymer gels are crosslinked polymer gels such as described in U.S. Pat. No. 5,048,609 "Selective Permeability Reduction in a Subterranean Hydrocarbon-Bearing Formation Using a Non-Selective Gel", issued Sep. 17, 1991 to Tackett, Jr. et al, and in U.S. Pat. No. 5,338,465 "Aqueous Gelable Composition With Delayed Gelling Time", issued Aug. 16, 1994 to Lockhart et al. Both these patents are hereby incorporated in their entirety by reference. Generally, the crosslinkable polymer is a carboxylate-containing polymer such as an acrylamide-containing polymer and the like. Of the acrylamide-containing polymers the most preferred are polyacrylamide, partially hydrolyzed polyacrylamide, copolymers of acrylamide and acrylate, and carboxylate-containing terpolymers of acrylate. The polyacrylamide most useful in such applications has from about 0.1 percent to about 3 percent of its amide groups hydrolyzed. The crosslinking agent affects crosslinking between the carboxylate sites of the same or different polymer molecules within the gel. The crosslinking agent is preferably a molecule or complex containing a reactive transition metal cation. Exemplary crosslinking agents are compounds or complexes containing chromium acetate, chromium chloride, aluminum compounds, zirconium compounds, iron compounds and the like. Such polymers and crosslinking agents are considered to be well-known to those skilled in the art. These polymers, with a suitable amount of a crosslinking agent are typically prepared as an aqueous solution and injected into the subterranean formation. The aqueous solution containing the polymer and crosslinker is then allowed to react to form a gel in the formation.
The various categories of gel strength are defined in U.S. Pat. No. 4,770,245, issued Sep. 13, 1988 to Sydansk. This reference is hereby incorporated in its entirety by reference. The gels may vary from highly viscous to substantially solid or ringing gels.
When such gels are positioned in a formation it is desirable, in some instances, to remove all or part of the gel from the formation. Most commonly it is desired to remove the gel from the oil-bearing zones of the formation while leaving the water-bearing zones of the formation plugged.
Previously, gels have been removed from subterranean formations by the use of strong oxidizing materials which are less effective than other agents which may be used to break the gel by chelating or otherwise reacting with the crosslinking agent. Various materials have been proposed for use as chelating agents, but all are attended by certain drawbacks such as, for instance in the use of EDTA (ethylenediaminetetraacetic acid), it has been found that EDTA is effective with the substantially solid ringing gels but works poorly with the viscous gels typically formed by the higher molecular weight polymers. Various of the other chelating agents have been found to leave precipitates of the gel in the formation after breaking.
It is desirable that an improved composition be available for breaking such gels which may be used to break the gels so that they are removed from the formation without leaving a residue and so that the gels may be selectively broken in oil-bearing zones while leaving the water-bearing zones plugged.
Unless otherwise stated, all references to percentages (%) are to weight percent.